In a diesel engine, reduction of NOx. and particulate matter (PM) in exhaust gas has been demanded. In a conventional diesel engine, when a piston reaches a vicinity of an upper dead center in a compression stroke, fuel is injected by fuel injection valves into atomization in a combustion chamber where the atomized fuel is mixed with air taken in through a suction valve to produce a fuel-air mixture which attains its ignition temperature through compression of the combustion chamber to burn.
Conventionally, in order to reduce NOx. in exhaust gas in such diesel engine, injection pressure of the fuel in, for example, an initial stage of fuel injection is reduced to reduce an amount of air (oxygen) to be mixed with the fuel. As a result, vigorous combustion of the fuel in the initial stage of fuel injection is suppressed to prevent flame temperature from being increased, thereby suppressing production of NOx. in exhaust gas.
A conventional diesel engine is disclosed, for example, in Patent Literature 1. Also in the Patent Literature 1, injection pressure of fuel is reduced to prevent excessive oxygen from being introduced into the fuel to thereby suppress production of NOx. in exhaust gas.
[Patent Literature 1] JP 2003-65169A